“Uhh… Oh, look what the birds are doing now, Teena — flying back and forth between those two fields.”“I think one side looks like a whale jumping out of the water, and the other side looks like the tail end of a buffalo or something.”

“Well, I can’t argue with that. Look, though — the murmuration’s acting like it’s caught between two barriers of some kind. That reminds me of another rule. When you’re one of those tiny quantum things, it matters if you’re caught between barriers.”

“I’d want to be free so I could go wherever I want to.”

“Freedom’s nice for people but it must be boring for quantum things. The rule says that a particle that doesn’t have any barriers just goes in a straight line forever and ever. No stopping for lunch, never anyone to talk to, just traveling on and on.”

“Yeah, that’d be boring, all right. What’s the rule say for when there’s barriers?”

“It depends on the barriers, what their shapes are and how far apart they are. The general situation, though, is that there’s usually some forbidden regions, places where the particle can’t go.”

“Oooo, forbidden. So spooky. What happens to the particles who go there anyway? Does something catch them and do bad things to them?”

“You’ve been watching too many horror movies. No doing bad things but no trying to go into a forbidden area anyway. Physics particles don’t have choice in the matter — they just can’t enter those places. Almost can’t.”

“I heard ‘almost.’ Are you being sneaky?”

“No, just trying to keep things simple. There’s something called ‘tunneling,’ where a particle that’s on one side of a barrier can sometimes somehow get to the other side of the barrier without going through it. It’s one of the big puzzles in quantum mechanics.”

“Can’t it climb over, like I climb over fences? (Shh, don’t tell Mommy.)”

“I suspect she already knows, Mommies are good at that, and I’m sure she’s praying that you’re being careful about which fences to climb and how you do it.”

‘Just to make things more complicated, a lot of the forbidden places aren’t even where the barriers are.”

“Huh?”

“Like I said, it depends on the shape of the barriers. If you’ve got two that face each other, there could be a forbidden place maybe in the middle, or two forbidden places a third of the way from each side, or three or four, all the way up. And here’s a weird case that’s really important. Ready to stretch your brain?”